Enable reading of sub hourly forecasts

[andrew-MET]

For NetCDF, it is possible to express lead times as numeric - assumed to be hours, or as strings with a suffix for units - h = hours, m = minutes, s = seconds.

This needs to be generalized for all formats - should be achievable by moving the conversion of lead times to seconds up to read_grid()

Lead times should also be able to expressed in mixed units - e.g. "03h30m" etc. This will need some modification in the templating for lead times, and a new function to generate sequences of lead times with particular formats.